By definition, unit magnetization or more specifically, unit magnetic moment in the centimeter-gram-second (CGS) system is possessed by a magnet formed by magnetic poles of opposite sign and of unit strength, one centimeter apart. The magnetic moment may be expressed for example as units per gram of per cubic centimeter. Techniques for measuring the magnetic moment of a material generally fall into one of three broad classes, those involving the measurement of forces resulting when a sample of the material is placed in a non-uniform magnetic field, those involving the measurement of flux changes, and indirect methods.
A preferred vibrating sample magnetometer for measuring magnetic moment which is presently in world-wide use and measures the flux change when a sample is moved in a substantially uniform magnetic field is described in U.S. Pat. No. 2,946,948 of the present inventor. That patent shows the common oscllation of a sample being analyzed and a reference source. A detector such as an inductive coil pickup positioned adjacent to the vibrating sample detects the change of magnetic flux produced by the moving sample and a reference detector disposed adjacent to the reference magnet generates a signal with a known phase relationship to the motion of the sample. The output signal indicating magnetic flux change which is proportional to the sample magnetic moment and the reference signal are differentially combined to provide a null output or a difference signal.
Improvements of the vibrating sample magnetometer of the aforementioned U.S. Pat. No. 2,946,948 are described in U.S. Pat. No. 3,496,459 also of the present inventor. According to that patent, the output signals from the magnetometer are analyzed at a frequency which is an harmonic of the vibration frequency of the sample.
An alternating force magnetometer analogous to the vibrating sample magnetometer described above, but in which force is applied to a sample by subjecting the sample to a periodically varying magnetic field gradient, is described in a paper by R. Reeves, entitled, "An Alternating Force Magnetometer," page 547, Journal of Physics E: Scientific Instruments, 1972, Volume 5.
A rotating sample magnetometer is described in articles by P. J. Flanders, entitled, "Utilization Of A Rotating Sample Magnetometer," Review of Scientific Instruments, Volume 41, No. 5, pp. 697-710, May 1970, and in an article by Stephen J. Hudgens, entitled "Rotating Sample Magnetometer For Diamagnetic Susceptibility Measurements," page 579, Review of Scientific Instruments, Volume 44, No. 5, May 1973.
Motion of conductive materials in an inhomogeneous magnetic field as in the operation of the vibrating sample and rotating sample magnetometers mentioned above, and application of a varying magnetic field to a sample as in alternating force magnetometers generate eddy currents of various strengths which, in turn, produce magnetic flux changes in the region of the sample. Eddy currents are not generated in insulating materials under such conditions since there is no conductive path provided in a sample of insulating material. In general, only negligible eddy currents are produced in ordinary conductive materials moving in a relatively uniform applied magnetic field under normal circumstances. Eddy current effects are particularly significant, however, when a sample is highly conductive and the magnetic moment of the sample which is sought to be measured is relatively small. A relatively pure bulk metal sample or a dilute alloy at low temperatures produces relatively significant eddy currents due to its high conductivity. Superconducting materials can generate extremely high eddy currents when vibrated in non-uniform fields. Even in a uniform magnetic field, significant eddy currents are produced when the sample does not move parallel to the applied field. The eddy current effects may be substantially eliminated by reducing the sample size, by using a powdered sample or by employing a more homogeneous applied field. In many possible applications, however, it is undesirable to alter the configuration of the sample and not feasible to employ a more uniform field. Out-of-phase eddy current effects must therefore be overcome in order to provide accurate measurement of magnetic moment for highly conductive materials or for less conductive materials in non-uniform magnetic fields.
In addition to eddy currents, a number of other out-of-phase signal components may distort the output indication of magnetic moment. Out-of-phase components may result, for example, when (a) the applied field is swept and the change in flux over time has various frequency components; (b) when the sample moves along an axis which is not parallel to the magnetic field lines; (c) when the sample mounting has mechanical resonances; (d) when the motion of the sample is hindered by binding to the driving mechanism so that the sample drive is not perfectly sinusoidal; (e) when vibrational noise is detected; and (f) when a mechanical driver is mechanically coupled to the detection coils.